Norman Y. Zhou

Bio: Norman Y. Zhou is an academic researcher from University of Waterloo. The author has contributed to research in topics: Welding & Spot welding. The author has an hindex of 22, co-authored 94 publications receiving 1460 citations. Previous affiliations of Norman Y. Zhou include Tsinghua University & Center for Advanced Materials.

Femtosecond laser ablation of highly oriented pyrolytic graphite: a green route for large-scale production of porous graphene and graphene quantum dots

Abstract: Porous graphene (PG) and graphene quantum dots (GQDs) are attracting attention due to their potential applications in photovoltaics, catalysis, and bio-related fields. We present a novel way for mass production of these promising materials. The femtosecond laser ablation of highly oriented pyrolytic graphite (HOPG) is employed for their synthesis. Porous graphene (PG) layers were found to float at the water–air interface, while graphene quantum dots (GQDs) were dispersed in the solution. The sheets consist of one to six stacked layers of spongy graphene, which form an irregular 3D porous structure that displays pores with an average size of 15–20 nm. Several characterization techniques have confirmed the porous nature of the collected layers. The analyses of the aqueous solution confirmed the presence of GQDs with dimensions of about 2–5 nm. It is found that the formation of both PG and GQDs depends on the fs-laser ablation energy. At laser fluences less than 12 J cm−2, no evidence of either PG or GQDs is detected. However, polyynes with six and eight carbon atoms per chain are found in the solution. For laser energies in the 20–30 J cm−2 range, these polyynes disappeared, while PG and GQDs were found at the water–air interface and in the solution, respectively. The origin of these materials can be explained based on the mechanisms for water breakdown and coal gasification. The absence of PG and GQDs, after the laser ablation of HOPG in liquid nitrogen, confirms the proposed mechanisms.

A study on heat affected zone softening in resistance spot welded dual phase steel by nanoindentation

Abstract: The base metal (BM) and the heat affected zone (HAZ) of a resistance spot welded dual phase steel have been evaluated by nanoindentation hardness testing. Three different surface conditions have been explored on the BM for assessing the nanohardness response. Softening has been investigated along the sub-critical HAZ by making nanoindentations on individual phases such as ferrite and tempered martensite (TM) at various distances from the line of lower critical temperature Ac1. A broken appearance accompanied with sub-micron particles were consistently found on TM at 100 μm from the Ac1 line suggesting carbide precipitation along with partial recovery of martensite. The morphology of TM kept on changing while moving away from Ac1 towards the BM as the fraction of broken appearance was reduced and the sub-micron particles became finer. SEM observations resulted in good agreement with the nanohardness of the TM phase along the sub-critical HAZ. In contrast, microhardness results suggested the termination of tempering at a shorter distance with respect to Ac1 and hence a reduced extension of the softening region. The improved resolution for assessing softening through nanoindentation was due to the possibility of avoiding the contribution of the phase boundaries because of the smaller size of the indentation; this also permitted evaluation of TM at low peak temperatures far from Ac1 where early stages of tempering took place.

Controlling mechanical properties of additively manufactured hastelloy X by altering solidification pattern during laser powder-bed fusion

Abstract: Like other manufacturing processes, controlling the microstructure of additively manufactured parts is essential to reach the desirable mechanical properties. However, available reports on the control of as-build microstructure and mechanical properties of Ni-base superalloys during laser powder-bed fusion (LPBF) process are not comprehensive. This article aims at a systematic approach to study the effect of scanning strategies and build orientations on solidification patterns in the printed LPBF Hastelloy X parts. The as-built microstructure (grain size, texture) and mechanical responses (yield strength, ultimate tensile strength (UTS), and elongation) are also presented. Results reveal that the stripe unidirectional scan pattern leads to the largest grain size (>850 μm) with the lowest mechanical strength. These samples also exhibit the strongest crystallographic texture, resulting in a planar anisotropic mechanical response (~22 MPa difference in UTS). On the other hand, the stripe rotation scan strategy (67° rotation) leads to a randomly oriented and finer grain structure (~110 μm) with a higher UTS (~800 MPa) due to grain refinement observed in these samples. In addition, the aspect ratio of the columnar grain structure was observed to influence the mechanical response of these parts. UTS of horizontally printed parts were ~26% more than the vertical parts for the stripe scan strategy (67° rotation). However, changing the solidification pattern (stripe XY with 90° rotation) was observed to reduce this difference to ~18%. These findings can be used to tune the microstructure of as-built LPBF parts to obtain an optimal mechanical behaviour.

An investigation into the effect of process parameters on melt pool geometry, cell spacing, and grain refinement during laser powder bed fusion

Abstract: An applied energy density approach defined as the ratio of laser power and scanning velocity is often used as a guideline for selecting appropriate process parameters in laser powder bed fusion (LPBF). In this study, amongst the many variables related to input energy, we investigate the effectiveness of laser energy density (LED) on the melt pool geometry and microstructure of Hastelloy X single tracks produced by fixed LED values at different laser powers and scanning velocities. The results reveal that for a fixed LED, the higher laser power has a higher effect on the melt pool depth. In addition, compared to the scanning velocity, laser power has a higher influence on the melt pool geometry. Moreover, it is proposed that the finer cell structure observed in the melt pool of high laser power is due to the higher cooling rates. Finally, the higher number of new grains observed in melt pools created with higher laser power and fixed LED are likely due to the grain detachment caused by the increase in the partially melted particles.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Laser Synthesis and Processing of Colloids: Fundamentals and Applications

Abstract: Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPC-synthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials,...

Tunneling effect in a polymer/carbon nanotube nanocompositestrain sensor

Abstract: A strain sensor has been fabricated from a polymer nanocomposite with multiwalled carbon nanotube (MWNT) fillers. The piezoresistivity of this nanocomposite strain sensor has been investigated based on an improved three-dimensional (3D) statistical resistor network model incorporating the tunneling effect between the neighboring carbon nanotubes (CNTs), and a fiber reorientation model. The numerical results agree very well with the experimental measurements. As compared with traditional strain gauges, much higher sensitivity can be obtained in the nanocomposite sensors when the volume fraction of CNT is close to the percolation threshold. For a small CNT volume fraction, weak nonlinear piezoresistivity is observed both experimentally and from numerical simulation. The tunneling effect is considered to be the principal mechanism of the sensor under small strains.

Recent advances in photocatalysis for environmental applications

Abstract: Advanced Oxidation technologies (AOTs) are gaining attention as an effective waste water treatment methodology capable of degrading diverse spectrum of recalcitrant organic contaminants and microbes. Undoubtedly, photocatalysis is a promising AOT to alleviate the problem of water pollution. Despite recent research into other photocatalysts (e.g. ZnO, ZnS, Semiconductor-Graphene composites, perovskites, MoS2, WO3 and Fe2O3), titanium dioxide (TiO2) remains the most popular photocatalyst due to its low cost, nontoxicity and high oxidising ability. Moreover, titania photocatalysts can easily be immobilized on various surfaces and be scaled up for large scale water treatment. The current review aims to highlight recent advancements in photocatalytic AOTs with main emphasis on TiO2 photocatalysis. This review also discusses the use of TiO2 photocatalysis for water and waste treatment, treating contaminants of emerging concern (CECs), pesticides, endocrine disrupters (EDs) and bacteria using both UV and visible light irradiations. It was concluded that with efficient photoreactor configuration and further studies on the photocatalyst regeneration, TiO2 photocatalysis is a viable option for the reclamation of agricultural/irrigational waste water. Novel doped photocatalysts such as ZnS-CuS-CdS, carbon spheres/CdS, g-C3N4-Au-CdS, ZnS-WS2-CdS, C3N4-CdS and Pd-Cr2O3-CdS have also been discussed. Finally, the advances in the actively studied metal organic framework based photocatalysts that are emerging as effective alternate for metal oxide based photocatalysts is also discussed in detail.